Capturing Complex Rate‐Dependent Behaviors of Saturated Clays: A Fractional Consistency Kinematic Hardening Viscoplastic Approach

Saturated high plasticity clays show complex nonlinear, rate‐dependent, and hysteresis behaviors under non‐monotonic stress paths, requiring advanced mathematical constitutive equations for accurate description. Taking into account the inherent advantages of kinematic hardening mechanisms in simulating complex stress histories, this paper presents a novel fractional order kinematic hardening viscoplastic model for describing complex rate‐dependent features of saturated high plasticity clays: first, the modified isotach viscosity is extended into general loading conditions to consider both loading and unloading rate effects; second, a combined rate‐dependent isotropic‐rotational‐kinematic hardening law is built through current and conjugate stress points res the non‐intersection of two surfaces and smooth transition; third, a stress‐fractional operator is defined to represent the non‐orthogonal plastic flow direction in the proposed model; fourth, based on the consistency condition on the bubble surface, the increment form of stress‐strain‐strain rate relationship can be formulated and implemented into a finite element code. Parametric analyses are then adopted to demonstrate the model's capabilities under different loading paths. Finally, three different saturated clays, namely natural Boom clay, Hong Kong marine deposits, and an Earth dam core compacted clay, are employed to validate the model's effectiveness and performance via various rate‐dependent non‐monotonic element test results.